The enzymes responsible for repairing DNA damage also function in several biologically significant pathways such as DNA replication, genetic recombination, and mutation fixation; and are thus directly implicated in the oncogenic process. Not only is little known about the repair of ionizing radiation-induced damages in DNA, but the chemical nature of these damages remains to be elucidated. The proposed research would be undertaken in an effort to define the molecular mechanisms involved in the repair to X-ray-induced damages in DNA using T4 bacteriophage and its Escherichia coli host a model systems. To this end, the DNA synthetic patterns of the recombinational repair deficient T4 x and y mutants have been examined, and the existence of aberrant DNA replicative intermediates established. An attempt will be made to isolate the products of the x and y genes by a complementation assay. Further, the E. coli X-ray endonuclease will be purified, and its properties delineated. Lastly, irradiated phage DNA will be repaired in vitro by purified enzymes of known specificities, and assayed for biological activity by transformation or transfection.